91
The Journal of Experimental Biology 210, 91-96
Published by The Company of Biologists 2007
doi:10.1242/jeb.02627
Chemical versus mechanical bioerosion of coral reefs by boring sponges – lessons
from Pione cf. vastifica
A. Zundelevich1,*, B. Lazar2 and M. Ilan1,†
1
Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel and 2Institute of Earth Sciences, The Hebrew
University of Jerusalem, Jerusalem 91904, Israel
*Formerly published as A. Peretzman-Shemer
Author for correspondence (e-mail: [email protected])
†
Accepted 31 October 2006
Summary
Bioerosion by boring sponges is an important
estimated from the total amount of CaCO3 chips produced
mechanism shaping the structure of coral reefs all around
over the same time interval. The measured bioerosion rate
the world. To determine the excavation rate by boring
of P. cf. vastifica was 2.3·g·m–2·sponge·day–1, showing
sponges, we developed a system in which chemical and
seasonal but not diurnal variations, suggesting that the
mechanical boring rates [calcium carbonate (CaCO3)
zooxanthellae harboring the sponge have no effect on its
dissolution and chip production, respectively] were
boring rate. The experiments indicated clearly that per
measured simultaneously in experimental tanks containing
each mass of chips that P. cf. vastifica produces during its
reefal rock inhabited by a boring sponge. Pione cf. vastifica
boring activity, it dissolves three masses of reef CaCO3
(Hancock 1849) was chosen as a model species to study the
framework. Assuming that some additional boring sponges
erosion rate of boring sponges. It is an abundant species in
can use a similar strategy of bioerosion, these findings
the coral reefs of the Nature Reserve Reef, Elat, Gulf of
suggest that chips, the most obvious erosion products of
Aqaba, northern Red Sea, reaching maximum abundance
boring sponges, represent only a small fraction of boring
at 25–30·m. The rate of chemical bioerosion was
sponge bioerosion capacity.
determined from the increase in tank-seawater alkalinity
over time, and the mechanical bioerosion rate was
Key words: Porifera, excavation, coral reef, dissolution, Clionaidae.
Introduction
The structure and form of the highly complex coral reefs are
mainly the result of the interaction between two processes:
construction and destruction. Reef growth is mainly attributed
to skeletal deposition of organisms having a calcium carbonate
(CaCO3) skeleton, such as stony corals, molluscs, polychaetes
and crustose coralline algae (Loya, 1990). The agents of
destruction are biological, physical and chemical, and in many
cases their effect on the erosion is synergistic (Hutchings,
1986). Biological erosion (bioerosion) as a general term refers
to the destruction and removal of consolidated minerals of
lithic substrate by the direct action of organisms (Neumann,
1966). The most common agents of bioerosion are fish (Risk
and Sammarco, 1982), sea urchins (Mokady et al., 1996), algae
(Kobluk and Risk, 1977), molluscs (Londono-Cruz et al., 2003;
Hutchings et al., 2005), polychaetes and sipunculids (LondonoCruz et al., 2003) and sponges (Holmes et al., 2000; Hutchings
et al., 2005). Physical erosion is caused by wave movement,
storms, etc. Chemical erosion involves dissolution of the
CaCO3 framework and is mainly mediated by biological
activity, either by metabolic acid production or by excretion of
ligands and enzymes. The organisms may also contribute to the
erosion through their physical boring and scraping activity on
the substrate.
Boring sponges, mostly from the family Clionaidae,
generally dominate the bioeroder community (Risk et al., 1995;
Calcinai et al., 2000). Whereas many sponges are chemically
or mechanically defended, some that have no such defenses
may have the competitive advantage of using a substrate that
other organisms cannot use. One such strategy is to bore into
the carbonate substrate that is out of reach of most predators,
with the additional advantage of using a space unavailable to
their competitors. The activity of boring sponges has farreaching ecological and economic effects: they can host many
organisms inside their water channels; affect settlement of new
organisms by changing availability of reef areas; shape the
morphology and affect the strength of the reef framework (Risk
and Muller, 1983; Neumann, 1966); influence the alkalinity and
the dissolved silica of reef-water chemistry; infect cultured
clam, oyster, or abalone populations in marine farms (Rosell et
al., 1999; Fromont et al., 2005); and bore into the structure of
piers and water breakers (Warburton, 1958). Boring sponges
THE JOURNAL OF EXPERIMENTAL BIOLOGY
92
A. Zundelevich, B. Lazar and M. Ilan
have been used for paleoenvironmental reconstruction, for
example, by measuring the size of bore holes in reefs from the
geological record (Edinger and Risk, 1996).
Boring sponges have developed a unique cellular means of
penetrating the substrate (see Bergquist, 1978; Pomponi, 1980).
The sponge attaches itself onto the substrate and then penetrates
by using etching cells to separate CaCO3 chips from the
substrate (size range, 15–85·␮m). Chip production constitutes
the so-called ‘mechanical boring’ of sponges; however,
detachment of chips from the hard substrate requires the use of
‘chemical boring’ as well. A groove is chemically etched,
isolating a chip from the substrate. It is then mechanically
transferred out of the sponge body via the water channels.
Rates of bioerosion depend on several biotic and abiotic
factors, including nutrient and food availability, temperature
(Risk et al., 1995; Hill, 1996; Holmes et al., 2000), physiological
state of the organism (Rützler, 1975) and the density and type of
substrate in which the organism bores (Hutchings, 1986).
The sponge Pione vastifica (Hancock 1849) has a
cosmopolitan distribution (Warburton, 1958). There is
evidence for its distribution in west and east Mediterranean
(Bromely et al., 1990; Rosell and Uriz, 2002), Barbados
(Holmes, 2000), the Red Sea, Mozambique and the Seychelles
(Calcinai et al., 2000). Because of its worldwide distribution,
there has been a genuine misconception regarding its true
definition (Rützler and Stone, 1986), and recently it has been
relocated to the genus Pione within the family Clionaidae
(Rosell and Uriz, 1997). The sponge hosts many symbiotic
organisms, such as zooxanthellae (dinoflagellates) and
Polydorella smurovi (Tzetlin and Britayev, 1985) (Polychaeta),
that live inside its tissue or on its surface, respectively. Despite
its high abundance and distribution over a wide depth range on
Red Sea reefs, the bioerosion activity of P. cf. vastifica has
never been studied and quantified.
The aim of the present study was to determine the erosional
impact of this sponge on the coral reef as a model for the boring
sponge erosion rates. We used P. cf. vastifica despite its relatively
low abundance (as compared with the abundance of other species
of boring sponges) in the studied reefs because of its encrusting
(␤) growth form. This growth form facilitates measurements of
chemical versus mechanical boring activity (see below), whereas
the papillate (␣) growth form of another co-occurring boring
species is more complicated for such measurements.
We studied P. cf. vastifica distribution and abundance in the
coral reef of Elat, measured its bioerosion rate and determined
the relative proportion of chemical versus mechanical boring
mechanisms. The sturdiness of this species facilitated the
development of our laboratory experiments and analytical
methodology. The experimental set-up we developed for the
study can also be easily adapted for other sponge species.
Materials and methods
Field collections and sponge census
The research was conducted in the Nature Reserve Reef
(NRR), northern Gulf of Aqaba, Red Sea (31°25⬘N; 34°53⬘E),
a fringing reef located south of the city of Elat, Israel. Samples
of reef rock heavily inhabited by individuals of the sponge
Pione cf. vastifica were collected by SCUBA diving using a
chisel and hammer. The samples were transferred for
acclimation into aquaria at the H. Steinitz Marine Biology
Laboratory (The Interuniversity Institute for Marine Sciences,
Elat, Israel).
Species abundance was estimated using 10⫻1·m belt
transects (Loya, 1972). Arbitrarily placed transects (N=15–20)
were performed at depths of: 10, 15, 20, 25 and 30·m. The
number of individual sponges and their surface area was
recorded for each belt transect. The surface area of each sponge
was estimated by covering it with an equivalent surface of
aluminum foil and converting the aluminum foil mass (MAl; g)
to sponge surface area (Asponge; cm2) using the calibration
curve: Asponge=227⫻MAl. Analysis of variance (ANOVA) and
nonparametric a posteriori LSD tests were used to analyze the
results.
Laboratory experiments
Three to five reef rock samples (formerly Porites sp. corals,
~15·cm diameter), heavily inhabited by Pione cf. vastifica
sponges, were collected from 20·m depth once every three
months, and were placed for a 24·h acclimation period into
Plexiglas containers filled with 2.5·l of filtered (0.2·␮m)
seawater (total N=24). As a control we used dead coral
skeletons (Porites sp. maintained out of the water for several
months), uninfected by any boring organism, that were
immersed in filtered seawater for 48·h prior to the experiments.
Prior to immersion, the inhabited rocks and the control
fragments were gently cleaned with a brush and washed three
times to remove all the epibionts that lived on the substrate and
which could affect the water composition. The experiments
started after replacing the seawater in the Plexiglas containers
and lasted for 24·h. During the experiments the containers were
aerated and covered to minimize water evaporation, which can
change water condition and composition.
Determination of chemical boring rate
Rate of chemical boring was determined by measuring
changes in total alkalinity (AT) in the seawater during the
experiments. Total alkalinity of ‘normal’ seawater is defined as
(Stumm and Morgan, 1981):
AT = [HCO3–] + 2[CO3–2] + [B(OH)4–] + [OH–] – [H+] , (1)
which in effect is the number of equivalents of strong acid added
to a seawater sample in order to reach the H2CO3 endpoint. In
our experiments, changes in total alkalinity resulted almost
entirely from precipitation or dissolution of CaCO3 (Lazar and
Loya, 1991). The molar amount of CaCO3 dissolved by the
sponge was half of the observed AT increase in the experimental
aquarium. Effects on total alkalinity other than CaCO3
dissolution or precipitation may result, for example, from AT
decrease due to aerobic respiration and production of HNO3, and
the opposite effect may result from photosynthesis and NO3–
assimilation. In our experimental system these non-conservative
THE JOURNAL OF EXPERIMENTAL BIOLOGY
A sponge chemical and mechanical boring
effects on AT were negligible. AT was measured by
potentiometric titration and by fitting a Gran function. The mass
of CaCO3 dissolved by the sponge, M(CaCO3) (in g), was
calculated from the change in total alkalinity [⌬AT (eq·kg–1)]
during the experiment (AT at the end of the experiment minus AT
at the beginning, ~24·h earlier) as follows:
M(CaCO3) = 0.5 (mol·eq–1) ⫻ ⌬AT ⫻ 100 ⫻ Vsw ⫻ ␳sw , (2)
where 100 is the molecular mass of CaCO3; Vsw is volume (l)
of seawater in the experimental aquarium; and ␳sw is the
seawater density (~1.028·kg·l–1).
The boring activity was measured both during day time and
at night, since it has previously been shown that distribution of
some Cliona species correlated with light irradiance (LopezVictoria and Zea, 2005), and Cliona varians (Duchassaing and
Michelotti 1864) boring rate was correlated with zooxanthellae
abundance and light irradiance (Hill, 1996).
Determination of mechanical boring rate
At the end of each experiment the aquarium was stirred, the
CaCO3 chips produced by the sponge and settled on the bottom
were resuspended and the seawater was filtered through a glass
fiber filter (3·␮m) to collect all the chips. The filter with the chips
was combusted at 450°C for 6·h to remove all organic matter.
After cooling, the residues were passed through 200·␮m and
100·␮m sieves, removing large particles, assuming that the
sponge-produced chips lie within a smaller fraction <100·␮m
(see above). The filtered chips were cooled to –70°C, freezedried to remove any traces of water and weighed.
Electron microscopy
Small fragments (~2⫻3⫻3·mm) of rocks containing sponges
were gently broken using sharp knives as chisels. The samples
were immersed for 24·h in diluted sodium hypochlorite. The
clean fragments were rinsed three times for 30·min in distilled
water and then three times for 30·min in ethanol (100%). The
samples were then air dried and glued on stubs for examination
with a scanning electron microscope. The fragments were
critical-point dried and sputtered with gold. The preparations
were viewed with a JEOL JSM 840A SEM-microscope
(Tokyo, Japan).
Clean sponge spicules for electron microscope examination
were obtained by decalcifying sponge samples (immersing the
samples overnight in diluted sodium citrate and formic acid)
and digesting the tissue (12·h immersion in sodium
hypochlorite). The residues were rinsed three times in water
and three times in ethanol (100%), mounted on stubs, gold
coated and inspected on a JEOL JSM 840A SEM-microscope.
The length (N=40) and width (N=25) of complete spicules from
each type were measured.
Results
Sponge identification and description
The sponges studied here were tentatively identified as Pione
cf. vastifica based on the brightly orange-red colors of the
93
sponge and the spicule types. The spicules found within this
sponge (Fig.·1) were of three types: tylostyles to subtylostyles
(length: range 155–241·␮m, mean 230±30·␮m; width: range
2.0–4.0·␮m, mean 3.6±0.6·␮m), acantho microxea (length:
range 77–125·␮m, mean 103±13·␮m; width: range
2.0–3.5·␮m, mean 2.8±0.4·␮m) and acantho microrhabds
(length: range 6.3–10.0·␮m, mean 8.5±1.1; width: range
2.0–4.0·␮m, mean 2.5±0.5). However, the clionaid’s
encrusting growth form, where the sponge covers the dead
surface skeleton of usually massive corals, has not been
reported for P. vastifica (Schönberg, 2002a), and hence the
current identification as P. cf. vastifica indicating its affiliation
with this species complex.
Sponge distribution in the reef
In the depth range of this study (down to 30·m depth), the
abundance of P. cf. vastifica increased linearly with depth by
~2 individuals per 10·m2 per 10·m depth (Fig.·2). This finding
was corroborated by an ANOVA test, which showed a
significant difference in abundance between the various depths
(P<0.001), and an a-posteriori test, which revealed three main
depth-related abundance groups (10, 15 and 20, and 25 and
30·m) in which the abundance increased with depth.
Surface area of individual sponges varied at all depths
Fig.·1. Pione cf. vastifica spicules and erosion scars. (A) All three
types of spicules. The longest one is a tylostyle. (B) An acantho
microxea. (C) An acantho microrhabd. (D) Wall of sponge excavation
in coral, covered with erosion scars. (E) A few erosion scars of
different shapes. Of note are the projections in the center of some of
them, where no substrate dissolution occurred. (F) A single erosion
scar with a small projection in the middle. Signs of penetration beneath
the projection are evident in its perimeter (arrowheads). Arrow
indicates the area assumed to have been chemically removed.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
94
A. Zundelevich, B. Lazar and M. Ilan
0.7
Dissolved CaCO3 (μg cm–2 day–1)
Abundance (individual m–2)
0.8
y=0.18x
r 2=0.82
0.6
c
0.5
c
0.4
0.3
b
b
0.2
0.1
a
0
0
10
20
250
*P<0.05
*
200
N=3
150
100
N=5
50
N=3
N=5
N=3
0
August November March
30
Depth (m)
April
June
Month
Fig.·2. Abundance of Pione cf. vastifica at different depths (mean ±
s.d.). a, b and c show statistically different groups (ANOVA and LSD
a-posteriori test; P<0.001).
Fig.·4. Chemical boring rate of Pione cf. vastifica in different months
(mean + s.d.), (ANOVA, P<0.05).
ranging from 20 to 1000·cm2. No significant difference was
found in a one-way ANOVA between sponge sizes at different
depths, and the mean sponge area was approximately 400·cm2
with only a small number of small individuals (Fig.·3).
by P. cf. vastifica showed the smooth erosion scars (length:
range 36–97·␮m, mean 59±18·␮m; width: range 17–73·␮m,
mean 40±13·␮m; N=15) to be polygonal to oval (often round)
in shape (Fig.·1). Closer observation revealed that the
excavated pit frequently had a small projection in the middle
(Fig.·1E,F), suggesting that the entire surrounding of this
projection had already been dissolved.
Sponge chemical bioerosion
Rate of chemical boring was measured from three to five
individual sponges for five different months, at water
temperatures ranging from 28.1°C in August to 21.5°C in
November (Fig.·4). In a one-way ANOVA test, a significant
difference was found between the rates of chemical boring
during these months (P<0.05). When analyzed by an LSD aposteriori test, the highest boring rate was measured in
November, in which the amount of CaCO3 dissolved by the
sponges was 180±30·␮g CaCO3·cm–2 sponge·day–1, and the
lowest rate was during March, with a rate of 30±4·␮g
CaCO3·cm–2·sponge·day–1. The mean chemical bioerosion rate
(calculated by annual integration) was 74±16·␮g CaCO3·cm–2
sponge·day–1, with no significant difference (according to a
paired t-test) between day and night (Fig.·5).
Discussion
Remarks on the species
The spicule types and the sponge color support its placement
within the genus Pione close to the P. cf. vastifica (Hancock)
species complex. However, we have previously found that this
Red Sea P. cf. vastifica always harbors photosynthetic
symbiotic algae (Beer and Ilan, 1998; Steindler et al., 2001).
Thus, the encrusting (␤) growth form and the presence of
symbiotic algae do not fit this species (Rützler, 2002a).
Recently, material from another previously recognized P. cf.
vastifica from Western Australia was reassigned to P. velans
(Fromont et al., 2005). Further systematic treatment of the
Sponge mechanical bioerosion
Electron microscope examination of the substrate excavated
Mean sponge surface area (cm2)
–1
Dissolved CaCO3 (μg CaCO3 h )
350
900
800
700
600
500
400
300
200
100
0
N=10
N=23
10
N=17
20
Depth (m)
30
Fig.·3. Surface area of individual Pione cf. vastifica at different depths
(mean + s.d.).
300
250
200
N=5
N=5
Day
Night
150
100
50
0
Fig.·5. Pione cf. vastifica chemical boring rate during the day and at
night (mean + s.d.).
THE JOURNAL OF EXPERIMENTAL BIOLOGY
A sponge chemical and mechanical boring
Sponge abundance and distribution
The abundance of Pione cf. vastifica was found to increase
linearly with depth in the range of 10–30·m (Fig.·2), with no
difference in individual size at the different depths. The increase
in population density was two individuals per 10·m2 for a depth
interval of 10·m (the slope of the trend-line in Fig.·2). No transects
were performed in the depth range of 0–10·m because P. cf.
vastifica abundance in this range was very low and the sponge
was found mainly in deeper waters. No systematic transects were
performed below 30·m due to logistical difficulties, but sporadic
dives indicated that sponge abundance below 30·m depth was
high. Such a pattern of population distribution could be the result
of several pre- and post-settlement processes that were not
examined in the course of the present study.
The scarcity of small individuals in the reef is rather
puzzling, especially because individual sponge tissue was
found to contain oocytes (A.Z., unpublished). This may suggest
a long period with no recruitment prior to the present study or
that young individuals grow very fast and after reaching a
certain size (>100·cm2) their growth rate slows substantially.
Alternatively, it might be that the smaller individuals grew
within the substrate and took the encrusting growth form only
after reaching a larger size. Since in the present study we
concentrated on the encrusting growth form, a situation as
described above would have been overlooked.
Sponge bioerosion rates
Many studies have examined boring sponges and their
activity because of their unique and important role in the
shaping of hard-bottom environments (e.g. Rützler and Rieger,
1973; Acker and Risk, 1985; Thomas, 1996). Most of these
studies, however, measured total CaCO3 disappearance and
frequently focused on the mechanical boring rates, i.e. the
production of CaCO3 chips, while guessing the amount of the
chemical boring rate (Warburton, 1958) or calculating (Rützler
and Rieger, 1973) it to be 10–2% of the total bioerosion,
respectively. The present study is the first to simultaneously
quantify both the chemical and mechanical bioerosion rates.
The mean amount of CaCO3 that was chemically removed from
the substrate was 260·g·m–2·sponge·year–1 (Fig.·4), which is
approximately three times more than that mechanically eroded
by the same sponge over the same period (80·g·m–2
sponge·year–1). The chemical and mechanical erosion rates
correlate positively (Fig.·6) with a chemical/mechanical boring
rate ratio of ~3, reflecting dissolution of three volumes of
CaCO3 during production of one volume of chips. Indeed, the
pattern of the erosion scars seen in pits with incomplete chip
removal (Fig.·1D-F) indicates that most of the scar has been
chemically dissolved, which has not been previously noted in
any other bioeroding sponge. The total bioerosion rate
(chemical plus mechanical) is 340±170·g·m–2·sponge·year–1,
10
Mechanical boring rate
(mg CaCO3 day–1)
present species from the Red Sea, such as a comparison with
P. mussae (Keller, 1891), will reveal its proper assignment. In
this paper, however, for lack of strong counter-characteristics,
this species is still called P. cf. vastifica.
95
y=0.29x
r 2=0.70
8
6
4
2
0
0
5
10
15
20
25
30
Chemical boring rate (mg CaCO3 day–1)
Fig.·6. Pione cf. vastifica rates of chemical boring versus mechanical
boring by individual sponges (mean ± s.d.).
which is equivalent to sponge bioerosion per reef area of
68±34·g·m–2·year–1. Boring activity during November was
significantly higher than during the rest of the examined
months (Fig.·4). The total erosion of the reef by this Pione
species is estimated to be rather small compared with clionaids
(0.2–23.8·kg·m–2·year–1 results given per sponge surface area
and not reef area) and other bioeroders (11–724·g·m–2·year–1
for sea urchins) reported from the same reef and from reefs
elsewhere [see Schönberg (Schönberg, 2002b) and Mokady et
al. (Mokady et al., 1996), respectively].
The large variability between studies in the measured sponge
erosion rates may stem from: (1) Variability in type and density
of the substrate. We used skeletons of dead Porites sp. coral
whereas other studies used other coral species and much denser
bricks of CaCO3 or skeletons of molluscs, as reviewed, for
example, in Schönberg (Schönberg, 2002b). The latter study
found that the measured CaCO3 erosion per unit volume
increased with substrate density. Substrate type may yield
erosion rate variability of up to six times the values for
measurements performed on the same sponge species in the same
locality [see, for example, table·4 in Schönberg (Schönberg,
2002b)]. (2) Differences in the species of sponges studied
(Schönberg, 2002b). The present research used only one species,
P. cf. vastifica. (3) Variability in sponge age. All specimens
studied in this study were mature individuals, which bore more
slowly, whereas other studies used young sponges or fragments
that bore faster (Rützler, 1975). (4) The length of the sponge’s
water canals. It was shown that the boring activity decreases
when the water canals pass a certain threshold length (Neumann,
1966). (5) Variability in nutrient levels. It was argued that high
nutrient levels stimulate boring rate in sponges (Holmes, 2000).
The surface water of the northern Gulf of Aqaba, northern Red
Sea, is depleted of nutrients during summer and may contain
relatively high nutrient levels during spring. (6) Variability in
water temperature. The sponge boring activity that might be
temperature-dependent (Rützler, 2002b) is low in the cooler
water of the northern Gulf of Aqaba (sea surface temperature
ranges of between 20°C and 28°C).
THE JOURNAL OF EXPERIMENTAL BIOLOGY
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A. Zundelevich, B. Lazar and M. Ilan
The results of this study indicate that the chemical boring rates
of P. cf. vastifica are constant during a diurnal cycle, showing
no change between night and day (Fig.·5). Light intensity, the
daytime net photosynthesis of the symbiotic zooxanthellae and
the night-time net respiration of the sponge were apparently not
reflected by a change in the boring rate of P. cf. vastifica. This
finding is in contrast to the results of an earlier study showing
that the boring rates of the sponge Cliona varians forma varians
correlated with zooxanthellae abundance and light irradiance
(Hill, 1996). However, it was shown that for the sponge species
studied here, P. cf. vastifica, the zooxanthella abundance did not
correlate with light intensity (Steindler et al., 2001), supporting
the findings of the present study.
In summary, the present study demonstrated the feasibility of
simultaneously measuring chemical and mechanical boring rates.
The combined chemical and mechanical boring rate of the
sponge P. cf. vastifica in the reefs of the northern Gulf of Aqaba
was estimated to be 70·g·m–2·year–1, which is ~1–2% of the total
growth (calcification) rate on these reefs (Barnes and Lazar,
1983). The abundance of P. cf. vastifica in these reefs is much
lower than that of other clionaid sponges. Therefore, the actual
role of boring sponges in northern Red Sea reef bioerosion is
most probably much larger than the rate cited above.
We thank A. Daya, E. Brokowitch and L. Steindler for their
assistance during the dives. The help of O. Mokady and A.
Ariel with the alkalinity titrations is highly appreciated. The
staff of the H. Steinitz Interuniversity Marine Institute, Elat,
and especially T. Rivlin and M. Dray, assisted in the research.
We are grateful for the help of S. Shefer and Y. Delarea in the
preparation of samples and work with the electron
microscope. We thank the Shilo Minerva Center for Marine
Biogeochemistry. The remarks of anonymous reviewers
helped to improve the work and the manuscript.
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